EventDetector.java

/* Copyright 2002-2023 CS GROUP
 * Licensed to CS GROUP (CS) under one or more
 * contributor license agreements.  See the NOTICE file distributed with
 * this work for additional information regarding copyright ownership.
 * CS licenses this file to You under the Apache License, Version 2.0
 * (the "License"); you may not use this file except in compliance with
 * the License.  You may obtain a copy of the License at
 *
 *   http://www.apache.org/licenses/LICENSE-2.0
 *
 * Unless required by applicable law or agreed to in writing, software
 * distributed under the License is distributed on an "AS IS" BASIS,
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 * See the License for the specific language governing permissions and
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package org.orekit.propagation.events;

import org.orekit.propagation.SpacecraftState;
import org.orekit.propagation.events.handlers.EventHandler;
import org.orekit.time.AbsoluteDate;

/** This interface represents space-dynamics aware events detectors.
 *
 * <p>It mirrors the {@link org.hipparchus.ode.events.ODEEventHandler
 * ODEEventHandler} interface from <a href="https://hipparchus.org/">
 * Hipparchus</a> but provides a space-dynamics interface to the
 * methods.</p>
 *
 * <p>Events detectors are a useful solution to meet the requirements
 * of propagators concerning discrete conditions. The state of each
 * event detector is queried by the propagator from time to time, at least
 * once every {@link #getMaxCheckInterval() max check interval} but it may
 * be more frequent. When the sign of the underlying g switching function
 * changes, a root-finding algorithm is run to precisely locate the event,
 * down to a configured {@link #getThreshold() convergence threshold}. The
 * {@link #getMaxCheckInterval() max check interval} is therefore devoted to
 * separate roots and is often much larger than the  {@link #getThreshold()
 * convergence threshold}.</p>
 *
 * <p>The physical meaning of the g switching function is not really used
 * by the event detection algorithms. Its varies from event detector to
 * event detector. One example would be a visibility detector that could use the
 * angular elevation of the satellite above horizon as a g switching function.
 * In this case, the function would switch from negative to positive when the
 * satellite raises above horizon and it would switch from positive to negative
 * when it sets backs below horizon. Another example would be an apside detector
 * that could use the dot product of position and velocity. In this case, the
 * function would switch from negative to positive when the satellite crosses
 * periapsis and it would switch from positive to negative when the satellite
 * crosses apoapsis.</p>
 *
 * <p>When the precise state at which the g switching function changes has been
 * located, the corresponding event is triggered, by calling the {@link
 * EventHandler#eventOccurred(SpacecraftState, EventDetector, boolean) eventOccurred}
 * method from the associated {@link #getHandler() handler}.
 * The method can do whatever it needs with the event (logging it, performing
 * some processing, ignore it ...). The return value of the method will be used by
 * the propagator to stop or resume propagation, possibly changing the state vector.</p>
 *
 * @author Luc Maisonobe
 * @author V&eacute;ronique Pommier-Maurussane
 */
public interface EventDetector {

    /** Initialize event handler at the start of a propagation.
     * <p>
     * This method is called once at the start of the propagation. It
     * may be used by the event handler to initialize some internal data
     * if needed.
     * </p>
     * <p>
     * The default implementation does nothing
     * </p>
     * @param s0 initial state
     * @param t target time for the integration
     *
     */
    default void init(SpacecraftState s0, AbsoluteDate t) {
        // nothing by default
    }

    /** Compute the value of the switching function.
     * This function must be continuous (at least in its roots neighborhood),
     * as the integrator will need to find its roots to locate the events.
     * @param s the current state information: date, kinematics, attitude
     * @return value of the switching function
     */
    double g(SpacecraftState s);

    /** Get the convergence threshold in the event time search.
     * @return convergence threshold (s)
     */
    double getThreshold();

    /** Get maximal time interval between switching function checks.
     * @return maximal time interval (s) between switching function checks
     */
    AdaptableInterval getMaxCheckInterval();

    /** Get maximal number of iterations in the event time search.
     * @return maximal number of iterations in the event time search
     */
    int getMaxIterationCount();

    /** Get the handler.
     * @return event handler to call at event occurrences
     * @since 12.0
     */
    EventHandler getHandler();

}